This invention relates to the separation of oil-well fluid mixtures, and concerns in particular the down-hole separation of the multi-phase oil/gas/water mixtures produced by an oil well.
Most oil wells produce what is basically a mixture of oil and/or gas together with water (usually in the form of xe2x80x9cbrinexe2x80x9d, carrying quite large amounts of dissolved minerals, mostly common salt). Somewhere in the production process the ingredients have to be separated, the oil/gas to be stored and then delivered to a refinery for subsequent treatment, the water to be disposed of (often by pumping it back into the ground, perhaps in some borehole neighbouring the one it was removed from so as to replace the liquid removed from the formation, and maintain the associated pressure). For the most part this separation takes place above ground, and once the oil/gas/water mixture has been pumped up to the surface, but there are a number of good reasons why it would be highly advantageousxe2x80x94regardless of what actually then happens to the waterxe2x80x94to effect the separation down-hole, even near the very earth formation out of which it has flowed into the borehole. Specifically, down-hole separation could result in a lighter liquid column within the borehole, it could permit a reduction in the separation equipment on surface, and it could facilitate the possibility of re-injecting the water directly down-hole, back into the formation (or a neighboring one) from which it originated. Other benefits of down-hole separation could be the easier separation before the mixture emulsifies, a reduced build-up of scale and corrosion, a minimizing of the risks from hydrogensulphide (a toxic gas often found in solution in oil well fluids), and a reduction in operating costs by reducing intervention in the well.
The invention relates to a method, and apparatus, for the separation of the water from the oil/gas in the well itself, down-hole. As explained in more detail hereinafter, the invention involves the combination of two ideas: the use of a gravity separator, and the utilization of controlling sensors positioned down-hole which identify the fluid components and then regulate the system accordingly. It is expected that this combination could be incorporated into an intelligent completion, in which the separated water is re-injected, resulting in an essentially water-free producing well.
In essence, the invention concerns a method of (and apparatus for) separating down-hole the two main components of an oil- (or gas-)well fluid, namely the oil/gas and the water, in which gravity is allowed to work on the mixture in a deviated section of pipe (a section that is not vertical, and then using physical separator meansxe2x80x94smaller pipes, baffles and the likexe2x80x94the gravity-separated components are lead to individual pumps that will bring them to the surface (or wherever: as noted, the water can be injected back into the formation). This is the gravity separator. The crucial point, though, is to organize the pumping correctly, so as to keep the unsettled/unseparated mixture away from the separator means xe2x80x9cinletsxe2x80x9d, and this requires that there must be positioned downhole, right next to the inlets, sensors/detectors that can distinguish between the two components, and then cause the pumping rates to be modified so as to keep the mixture away from the inlets, and thus keep the inlet feeds xe2x80x9cpurexe2x80x9dxe2x80x94the one or the other component, not the mixturexe2x80x94or at least xe2x80x9cpurerxe2x80x9d than the fluid actually issuing from the producing formation.
In one aspect, therefore, the invention provides a method of separating down-hole the main components (water and oil/gas) of a multi-phase oil- (or gas-)well fluid emanating from an underground formation into which a well has been drilled, in which method:
in a deviated, non-vertical section of the borehole the fluid is allowed to separate under gravity, aided by the flow regime imposed by the deviation, into separate streams or layers each being one (or mostly one) of its main components;
each stream is allowed to enter a conduit or pipe positioned to receive that stream only, and though which the component may be pumped away as required; and
component detector means are situated close to the effective mouth of each conduit/pipe, and are operatively linked to the relevant pump to alter the pump rate in dependence upon the actual nature of the material reaching the mouth.
In a second aspect the invention provides apparatus for use in a method of separating down-hole the main components (water and oil/gas) of a multi-phase oil- (or gas-)well fluid emanating from an underground formation into which the well has been drilled, which apparatus comprises, for location in a deviated, non-vertical section of the borehole within which the fluid is allowed to separate under gravity, aided by the flow regime imposed by the deviation, into separate streams or layers each being one (or mostly one) of its main components:
divider means by which the borehole is effectively divided into separate conduits or pipes each positioned to receive one respective fluid component stream only;
pump means associated with each such conduit/pipe and by which the relevant component may be pumped away as required; and
component detector means situated close to the effective mouth of each conduit/pipe, and operatively linked to the relevant pump to alter the pump rate in dependence upon the actual nature of the material reaching the mouth.
The invention concerns the down-hole separation of the main components of a multi-phase well fluid emanating from an underground formation into which the well has been drilled. These components will normally be oil and/or gas (the lighter) and water (the heavier). The proportions of oil/gas and water may vary very widely depending upon the field from which the oil/gas is coming. Indeed, the ratios vary widely not only between fields but also within a fieldxe2x80x94a range of likely ratios could be from 1:100 to 100:1 and still be valuable; the invention is useful for all of these.
Hereinafter the well fluid is for convenience referred to as though it were merely a two-phase oil/water mixture, resulting in two streams. This, it will be understood, is a handy simplification; in reality the fluid could be oil/water or gas/water, or even three-phase oil/gas/water (resulting in three streams).
In the first stage of the invention""s method the oil/water mixture emanating from the formation is passed into a deviatedxe2x80x94non-verticalxe2x80x94section of the borehole, and there the fluid is allowed to separate under gravity into two streams or layers each being one of its two main components. Oil being lighter than water, a mixture of the two will tend to separate if allowed to do soxe2x80x94if left alone, or moved only slowlyxe2x80x94with the oil rising to the top. In a horizontal pipe (borehole) the mixture would thus stratify, forming two quite distinct layers each moving along with the other but not mixing, whereas in a pipe/borehole at an angle to the verticalxe2x80x94for example, at 45xc2x0xe2x80x94there will be a tendency for the oil to move up along the upper side of the pipe while the water moves up along the bottom side (it is even possible for the water to try to move down along the bottom side of the pipe). The slip velocity between the several components plays a significant role in determining what angles of deviation give the best practical separation for the purposes of the invention, and it has been found that in general an angle of 40xc2x0 to 60xc2x0 to the vertical seems most satisfactory.
In the deviated, non-vertical section of the borehole the fluid is allowed to separate under gravity into two streams or layers each being one of its two main components. In a horizontal section it is evident that given the chance the fluid will stratify, the oil and water separating due to their differences in density, butxe2x80x94and perhaps surprisinglyxe2x80x94even in an angled section gravity can be used quite effectively to separate oil and water. In this type of separation the efficiency of the process is mainly controlled by the velocity of the fluids in the pipe and the slip velocity between the oil and the water. In classical design the oil bubbles rise freely in water at a slip velocity of about 10 cm/s. In angled deviated pipe sections, however, the process of separation can be accelerated; the peculiar velocity and oil concentration profile observedxe2x80x94a high velocity and high oil concentration at the top of the section and a low or even negative velocity and low oil concentration at the bottomxe2x80x94results in a much higher effective slip velocity between the oil and water (up to 80 cm/s) which increases with the deviation and reaches a maximum between 60xc2x0 and 80xc2x0 degrees. FIG. 1 of the accompanying Drawings (discussed further hereinafter) gives diagrams to illustrate the velocity and hold-up profiles of a multi-phase deviated flow. Various equations can be used to describe hold-up profiles, but for present purposes it is merely necessary to understand qualitatively that the water hold-up is higher at the lower side of the pipe.
In the invention the fluid is allowed to separate under gravity into two streams or layers each consisting predominantly of one of the fluid""s two main components (oil and water), and each stream is allowed to enter a conduit or pipe positioned to receive that stream only, and though which the relevant component may be pumped away as required. At its conceptually simplest the borehole above the separation point could be divided, by a partition wall roughly co-planar with its long axis, into two parts stretching the length of the hole, the wall being so orientated in the deviated section that one part is an xe2x80x9cupperxe2x80x9d part and the other a xe2x80x9clowerxe2x80x9d part. Clearly, if the oil/water mixture has been allowed to separate into two corresponding layersxe2x80x94an upper, oil layer and a lower, water layerxe2x80x94then, assuming a reasonable alignment, as the fluid is pumped out the upper, oil layer will enter and be pumped out through the upper part while the lower, water layer will enter and be guided through the lower part. There are many waysxe2x80x94many configurationsxe2x80x94in which this division of the borehole pipe into parts can usefully be achieved in practice, and a number are shown in and briefly discussed hereinafter with reference to the accompanying Drawings. One basic possibility is to have two separate pipes within the borehole, one (picking up the oil) above the other (picking up the water). Another is not to divide any one length of borehole into two xe2x80x9cparallelxe2x80x9d conduits but instead to divide the borehole into two serially-linked portions, with the stratified fluid entering at the division point and part going on up the borehole and the rest going on down the borehole (as discussed further hereinafter with reference to FIG. 9B of the accompanying Drawings). Instead of a partition, there may be employed a deflector to assist in this separation.
Each stream is allowed to enter a conduit or pipe positioned to receive that stream only, and though which the relevant component may be pumped away as required. The oil, of course, will be pumped up to the surface. The water may also be pumped to the surface, or it may be re-injected into the formations through which the borehole passes. Preferably the water is guided into a formation having a lower pressure than the producing formation. This low pressure formation can be a formation depleted in a previous production cycle. If the pressure differential between low pressure formation and the producing formation is sufficiently high, the water flow can be discharged without using a (second) pumping device. Of course, this aspect of the invention has considerable advantages as it reduces the number of downhole pumps or, at least, reduces the load on the pump.
Some examples of this latter are shown in and briefly discussed hereinafter with reference to the accompanying Drawings.
As is noted above, the crucial necessity in effecting the invention is to ensure not only that the oil well oil/water fluid mixture is allowed to separate under gravity into two streams or layers each being predominantly one of its two main components, but also that each stream is allowed to enter a conduit or pipe positioned to receive that stream only. This latter effectxe2x80x94each stream being allowed to enter the appropriate conduitxe2x80x94depends very much on the rate at which the oil well fluid is being pumped (and in particular on the relative rates of each separated component), for the faster it is pumped the less time it has to separate out, and the more likely it is that the fluid that reaches the inlet to/mouth of the conduit is not a single, separated components but is still a mixture. Of course, if the fluid is pumped much more slowly then the well simply doesn""t produce enough, and is inefficient. It is therefore important to monitor the situation, and optimally adjust the pumping speed to produce oil as fast as possible while still permitting adequate separation and not allowing a mixture to reach the conduit inlets.
This might seem a simple matter; observe what reaches the surface, and adjust the pumping rates accordingly. It is not simple, however, for under the ambient conditions a well constantly varies the rate at which it produces, and since the production zone in the well is likely to be several miles below ground, at such a distance, and with the time delay that it represents (as an example, at a speed of 1 m/s it may take as long as 30 minutes for well fluid to reach the surface), it is plainly not possible accurately to control the pumping rates so as to keep the feed to the conduits"" inlets a single component rather than an unseparated mixture. It is therefore essential to incorporate the pump-controlling sensor/detector equipment downhole, adjacent the inlets themselves. And thus oil-fluid-component detector means are situated close to the effective mouth of each conduit/pipe, and are operatively linked to the relevant pump to alter the pump rate in dependence upon the actual nature of the material reaching the mouth.
The detectors need to be placed close toxe2x80x94and preferably just insidexe2x80x94the conduits/pipes along which the fluid components are to be pumped away. In the case (discussed further hereinafter with reference to FIG. 9B) where the conduits are two opposed portions of borehole with a xe2x80x9cdeflectorxe2x80x9d separating them, the detectors should be adjacent the deflector.
The detectors utilized for this purpose may be any appropriate to the task, i.e., detectors that work well under the extreme conditions of temperature and pressure 20,000 ft(about 5 miles, or 4 kilometer) down an oil well borehole. One suitable type of detector is that known as a gradiomanometer, which with associated pressure gauges can detect the density of the liquid (and thus distinguish between the lighter oil and the heavier water). The measurements made using such detectors are an average of the density in a relatively long section of tubing, becoming even longer in highly deviated wells.
Another type of detector is the X-ray densitometer, wherein an X-ray source and a detector are placed at opposite sides of a section of pipe, and the attenuation of the radiation is used to calculate the density of the fluid in the pipe. A similar physical configuration of detector can be applied using an optical source and a photo-detector. Yet other varieties of detector may be suitable, including electromagnetic and ultrasonic detectors and impedance measurers.
Several embodiments of the invention are now described, though by way of illustration only, with reference to the accompanying schematic (and not to scale) Drawings.